With recent advancement of functionalities of compact information devices, such as cellular phones or PDAs (personal digital assistants), they are becoming capable of connecting to network or storing a larger amount of personal information, a great demand exist for enhancing the security performance of such devices. Conventional personal authentication techniques, such as those using passwords or ID (Identification) cards, can be adapted in order to ensure security at such devices.
However, since passwords or ID cards have a risk of being stolen, achieving a more reliable personal authentication technique (for authenticating that a user of a device is a true reregistered user) has been demanded.
For such a demand, a personal authentication by means of biometric information (biometrics) is highly reliable, thus meeting the above demand. This is especially convenient, when a fingerprint is used as biometric information.
When a personal authentication is performed using a fingerprint as biometric information, a fingerprint (i.e., a pattern constructing ridges that can contact a collection surface of a fingerprint sensor and valleys that cannot contact the surface) is collected as image information from a finger of an authenticated person using a capacitance type fingerprint sensor or an optical type fingerprint sensor.
Then, feature information (for example, position information of branch points or end points) is extracted from a foreground (for example, a ridge image) of the fingerprint image (hereinafter, also simply referred to as “fingerprint”), and the authenticated person is really who he is, is determined, i.e., a personal authentication is performed, by comparing the extracted feature information with registered feature information of the authenticated person that has been registered in advance.
A typical fingerprint sensor (hereinafter, sometimes referred to as a “flat surface type fingerprint sensor”) that collects a fingerprint image from an authenticated person generally includes a sensor surface (collection surface) that is generally wider than the size of a finger.
However, in recent years, in order to incorporate fingerprint sensors into compact information devices, such as cellular phones or PDAs, the size of a sensor surface is made smaller than the size of a finger. Alternatively, an entire image of a fingerprint is obtained by using a sensor surface that is smaller and thinner than the size of a finger by combining a plurality of partial images that are sequentially collected.
Particularly, there are surface type fingerprint sensors or sweep type fingerprint sensors that are smaller than the size of a finger (see Patent References 1 and 2 listed below, for example).
A sweep type fingerprint sensors has a small rectangular collection surface (sensor surface/imaging surface) that is significantly shorter than a length of a finger. A plurality of partial images of a fingerprint of a finger is sequentially collected by the fingerprint sensor while the finger is being shifted with respect to the collection surface or the collection surface (fingerprint sensor) is being shifted with respect to the finger. The entire fingerprint image of the finger is regenerated from the plurality of partial images that are collected.
From the regenerated fingerprint image, information on feature points (branch points or endpoints of ridges) is extracted and generated, and a personal authentication is performed on the basis of the information. Note that such a relative shift of a finger with respect to the collection surface is referred to as a “sweep” or “slide.”
Fingerprint sensing apparatuses having such sweep type fingerprint sensors are disclosed in Patent References 1 and 2 listed below, for example. In a sweep type fingerprint sensor, as described in Patent References 1 and 2 listed below, partial images of a fingerprint is sequentially captured by changing relative positions between the sensor and the finger. Both of the disclosures assume that partial images have overlapped portions when obtaining entire information of a fingerprint from the collected partial image of the fingerprint.
In addition, since a sweep type fingerprint sensor shifts a finger when capturing the fingerprint, a fingerprint that may not be obtained by conventional type fingerprint sensors against which a finger is pressed may be captured.
For example, generally, a fingerprint is collected at the distal end side from the distal joint (first joint). However, sweep type fingerprint sensors may capture a wider area, such as the distal joint or further proximal side (palm side), i.e., the area from the midpoint between the distal joint and the proximal joint to finger tip, or the area from the midpoint between the proximal joint and the middle finger joint to the finger tip, since sweep type fingerprint sensors can sequentially sense the finger while sliding the finger on the sensor. Especially, distal joints tend to be captured since they are adjacent to fingerprints.
There are techniques to extract such a finger tip area from a distal joint to the distal end side, i.e., an area of a fingerprint, and a technique to detect flexion creases in which the epidermis are concaved in grooves on the distal joint is disclosed in Patent References 3 to 5 listed below, for example.
Patent Reference 1: Japanese Laid-Open Patent Application No. H10-091769
Patent Reference 2: Japanese Laid-Open Patent Application No. 2003-208620
Patent Reference 3: Japanese Laid-Open Patent Application No. 2003-67749
Patent Reference 4: Japanese Laid-Open Patent Application No. 2006-72758
Patent Reference 5: Japanese Laid-Open Patent Application No. H09-134427
At a sweep type fingerprint sensor, as depicted in FIG. 23(a), a fingerprint is preferably collected while a user (authenticating person) sliding a finger from the starting position at which the area around the distal joint of the finger is pressed against a collection surface 100 of the sweep type sensor depicted in FIG. 23(a) toward the palm side (i.e., the right side in FIG. 23(a)) while contacting the finger on the collection surface (hereinafter, referred to as “sensor”) 100.
The user may capture the fingerprint as depicted in FIG. 23(b) while bending his finger from the status depicted in FIG. 23(a). Note that FIG. 23(a) is a diagram illustrating the position of the finger when a slide starts, and FIG. 23(b) is a diagram illustrating a position of the finger when the slide ends.
However, when the fingerprint is captured by the sensor 100 while the finger is bent, an imperfect fingerprint image may be obtained in which the width of the fingerprint contacting the sensor 100 is narrowed around the center of the fingerprint, as in FIG. 24.
That is, as depicted in FIG. 23(b), when a fingerprint is captured while bending a joint of the finger, only a part of fingers indicated by the reference symbols 101 and 102 contact the sensor 100. As a result, as depicted in FIG. 24, a fingerprint image 104 of an imperfect collection status having narrowing part 103 may be collected.
Such a fingerprint in the imperfect collection status is a not good fingerprint suited for a fingerprint authentication since the fingerprint may have a limited number of feature points that match to those in a registered fingerprint, which may make the user authentication difficult.
In addition, both of surface type fingerprint sensors and sweep type fingerprint sensors may face the same difficulties. An example includes unclear differences between ridges and valleys. If the epidermis is cornified or a part of the epidermis is peeled off for some reasons, such as an injury, a fingerprint in an imperfect collection status image may be collected in which differences between ridges and valleys in the fingerprint becomes locally unclear.
In this case, the unclear part may not be captured when the sensitivity of the sensor is adjusted to the sensitivity of the entire fingerprint, or, the sensitivity of the entire fingerprint may become improper when the sensitivity of the sensor is adjusted to the unclear part. In addition, enhancement processing by means of software may enhance noises and may generate a pattern that is different from the biometric information, making correction of the imperfect status difficult.
Note that conventional techniques may determine that the contact width is narrow for a part at which sensing of the fingerprint is locally difficult. Thus, in a fingerprint image 104 in the imperfect collection status as depicted in FIG. 24 may be collected by surface type fingerprint sensor when the pressing force of a finger against a fingerprint sensing surface is weak, or when the surface of the finger is not clean or when skin is not in a good condition.
As a result, when a fingerprint image 104 as depicted in FIG. 24 is collected, it is preferred that a collection status of the fingerprint image 104 is determined that the narrowing part 103 in the fingerprint image 104 is not a joint and the fingerprint image 104 is imperfect.
The technique in the above Patent Reference 3 detects a joint position by performing a Sobel conversion on an input image followed by application of a Gauss conversion, and performing DP matching a power value calculated by concentration square sum and a standard power value having an ideal shape for each horizontal line in the image.
In addition, the technique in the above Patent Reference 4 divides a fingerprint image into predetermined areas, extracts orientations of ridges, and extracts the position of a joint by the directions and arrangements of the ridges. The technique in the above Patent Reference 5 determines a projection histogram by concentration projection processing on a fingerprint image in the X direction, calculates minimum points as valley candidates and the certainty factors of the valley candidates from the projection histogram, and extracts a joint from the candidates having the highest certainty factor.
However, conventional techniques disclosed in Patent References 3 to 5 cannot distinguish between a joint position and imperfect positions for an imperfect fingerprint image 104 as depicted in FIG. 24 in which the width of the fingerprint contacting the sensor is narrowed around the center of the fingerprint. Thus, the narrowing part 103 may be erroneously detected as a joint position, or the imperfect narrowing part 103 cannot be detected.